This invention relates in general to a method of repositioning teeth for use in orthodontic treatment. Particularly, this invention relates to the use of orthodontic appliances for producing tooth movements. More particularly, this invention relates to the use of a plurality of elastic repositioning appliances for producing such tooth movements.
Orthodontic treatment is based on the principle that if prolonged pressure is applied to a tooth, tooth movement will occur as the bone around the tooth remodels. Bone is selectively removed in some areas and added in others. In essence, the tooth moves through the bone as it remodels, carrying its attachment apparatus with it as the socket of the tooth migrates. This attachment apparatus is a heavy collagenous supporting structure called the periodontal ligament (PDL) which attaches the tooth to the adjacent alveolar bone. The tooth remodeling is mediated by the PDL. Although the PDL is adapted to resist forces of short duration, prolonged force produces a different physiologic response, namely remodeling of the adjacent bone. Orthodontic tooth movement is made possible by the application of prolonged forces.
The simplest form of orthodontic movement is tipping. Tipping movements are produced when a single force is applied against the crown of a tooth. When this is done, the tooth rotates around its "center of resistance", a point at which resistance to movement can be concentrated for mathematical analysis. The center of resistance for a tooth is at the approximate midpoint of the embedded portion of the root, about halfway down the root. When the tooth rotates in this fashion, the PDL is compressed near the root apex on the same side as the force and at the crest of the alveolar bone on the opposite side. These areas account for only one-half the PDL area that could actually be loaded. Therefore, pressure in the two areas where it is concentrated is high in relation to the force applied to the crown. For this reason, forces used to tip teeth may be quite low and such movement may be easily achieved.
In many cases, mere tipping of the teeth is insufficient in completing orthodontic treatment. Translation, bodily movement of the tooth where the root apex and crown move in the same direction by the same amount, is often necessary. This may be accomplished by applying forces to the crown which create counterbalancing moments. Thus, the tooth would remain upright and move bodily. In this case, the total PDL area is loaded uniformly. Therefore, it is apparent that to produce the same pressure in the PDL and the same biologic response, twice as much force would be required for bodily movement as for tipping. To move a tooth so that it is partially tipped and partially translated would require forces intermediate between those needed for pure tipping and pure bodily movement.
For a number of reasons, including lowering the level of force applied to the teeth, at least part of the translation process is often accomplished by "tipping" and "uprighting." This technique is often referred to as the Begg approach or technique. Raymond Begg adapted current technology in the 1920's to produce the Begg appliance. The Begg appliance uses "stationary anchorage", in which movement teeth are allowed to tip while anchor teeth are allowed to only move bodily if at all. In the case of closing a premolar extraction site, anterior teeth would first be tipped distally. As a second step, the tipped teeth would be uprighted, moving the canine roots distally and torquing the incisor roots lingually. Using this technique, the optimum pressure for the anterior segment would be produced by about half as much force as if the anterior teeth were to be retracted bodily.
The Begg appliance and appliances using the Begg technique are typically bonded to the teeth. Bonding facilitates tooth movements by providing consistent points of purchase on the tooth for manipulation. During tipping, the crown is tipped in the direction of tooth movement. Interconnected bonded brackets or bands prevent the crown from substantially intruding the gingival line. The bonded appliances are then utilized to upright the tooth at its destination. However, many removable appliances, such as elastic repositioning appliances, do not have the benefit of bonding to manipulate the teeth. In these cases, teeth are commonly repositioned by manipulation of the crown directly. Points of purchase may be increased with the use of attachment devices on the crown. However, as a tooth becomes intruded during tipping, the portion of exposed surface area on the crown decreases. This diminishes the ability of an appliance, such as an elastic repositioning appliance, to "grab" onto a tooth for further manipulation. Therefore, the ability of accomplishing long, complicated movements in later stages of orthodontic treatment may be hindered. Further, translation of the tooth in the crown-first position is more difficult with the use of elastic repositioning appliances due to the slope of the crown. Force applied to the crown may serve to further intrude the tooth or may be reduced to a level of ineffectivity.
For these reasons, it would be desirable to provide alternative methods and systems for moving teeth. These methods should be compatible for use with any type of orthodontic appliance, both bonded and removably attachable. However, such methods may be particularly suitable for use with removable orthodontic appliances, including elastic repositioning appliances. In the case of elastic repositioning appliances, such methods and systems should also benefit the utilization of attachment devices, if available, and possibly lessen the need for additional attachment devices. At least some of these objectives will be met by the methods and systems of the present invention described hereinafter.